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Soil water potential

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C
Chou Bảo

The document summarizes soil water potential and its components. Soil water potential (ΨT) is equal to the sum of gravimetric potential (Ψz), osmotic potential (Ψs), matric potential (Ψm), and pressure potential (Ψp). Gravimetric potential depends on gravity and soil moisture content. Osmotic potential is due to dissolved salts. Matric potential restricts water movement through pore spaces. Pressure potential is positive in saturated soil. Together these determine if water will move within the soil from high to low water potential. The water content soils can hold is illustrated on a log scale, with sandy soil holding the least and clay the most at given potential values.

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ΨT = Ψz+Ψs+Ψm+Ψp WhereΨp = total potential Ψz = gravimetric potential Ψs = solute or osmotic potential Ψm = matric adsorption force Ψp = pressure due to external forces. Soil water potential
• Higher moisture above the level of water table in soil, gravity exerts a positive pulling force water move downward. • When higher in moisture content below soil profile and drier above profile, this would be a negative value, the water is going to be pulled up into a drier soil layer above. z Gravimetric potential
Osmotic potential • Measurement of ions (dissolved salt) which exert positive attraction for water • Negative value • Indirect measurement of attraction that ions have in soil water potential equation. • As soil salt content increases, more free ions available, larger negative number s
Matric potential • Taking energy away from soil water • Force restricting the movement of water through the pore spaces • In any soil less than saturation, the soil will be negative number • Potential energy of water is influenced by the matrix potential and will be negative energy. • Dry soil- larger negative number m
Pressure potential •The pressure potential will be positive in a saturated soil •In unsaturated soils, the pressure potential is, usually, considered zerop
ΨT = Ψz+Ψs+Ψm+Ψp • The total soil water potential equation will give us a value when we add up all of the effects of the gravitation, matrix and osmotic • It will give us a value which will be a negative number at anything less than saturation. • Water will move in soil profile from an area of high content to an area of lower content •ΨT Can be measured in kPa, bars or atmospheres
ΨT = Ψz+Ψs+Ψm+Ψp • Smaller negative number, the higher the water content in the soil pores • The larger the negative number, the lower content will be in the soil pores Water will moves from area of higher to lower concentration Water moves from area of a lower negative value forΨT towards to an area of a higher negative value forΨT. Or
Soil water potential log scale • The sandy soil holding considerably less water than the loam soil which holding correspondingly less amount of total water than the clay (fine textured soil) • In all case as soil water potential increases or becomes a larger negative number the soil will end up losing water regardless of whether it’s medium or fine texture soil • The best thing to use this graphic to illustrate is what type of soil moisture is remaining at a given soil water potential value.
At -0.1 bars/atmospheres to -1 bars/atmospheres: • The sandy soil is not holding very much water • The loam soil is holding an intermediate amount • The finer clay texture soil is holding the most water at that soil water potential • -1 bars/atmospheres is not a large negative number, it is high value and has a lot of usable water in the soil profile
At -10 bars/atmospheres • The water deficiency point for plants is usually around -10 bars/atmospheres and it is vary from one plant to another • It is fairly representative value that most of crop plants at least can take water out of soil profile without significantly damaging their yield potential • Everything’s going to this value it’s a combination of gravitational, matric and osmotic potentials
Conclusion of suitable value of soil water potential for plants • For all the practical purposes, the level of water being held between -0.1 to -10 bars/atmospheres is really usable water that being stored in the soil plant. • At -10 to -100 bars/atmospheres is very little water left in the soil profile. Rarely plants are able survive and utilize water from soils down to this level.

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Soil water potential

  • 1. ΨT = Ψz+Ψs+Ψm+Ψp WhereΨp = total potential Ψz = gravimetric potential Ψs = solute or osmotic potential Ψm = matric adsorption force Ψp = pressure due to external forces. Soil water potential
  • 2. • Higher moisture above the level of water table in soil, gravity exerts a positive pulling force water move downward. • When higher in moisture content below soil profile and drier above profile, this would be a negative value, the water is going to be pulled up into a drier soil layer above. z Gravimetric potential
  • 3. Osmotic potential • Measurement of ions (dissolved salt) which exert positive attraction for water • Negative value • Indirect measurement of attraction that ions have in soil water potential equation. • As soil salt content increases, more free ions available, larger negative number s
  • 4. Matric potential • Taking energy away from soil water • Force restricting the movement of water through the pore spaces • In any soil less than saturation, the soil will be negative number • Potential energy of water is influenced by the matrix potential and will be negative energy. • Dry soil- larger negative number m
  • 5. Pressure potential •The pressure potential will be positive in a saturated soil •In unsaturated soils, the pressure potential is, usually, considered zerop
  • 6. ΨT = Ψz+Ψs+Ψm+Ψp • The total soil water potential equation will give us a value when we add up all of the effects of the gravitation, matrix and osmotic • It will give us a value which will be a negative number at anything less than saturation. • Water will move in soil profile from an area of high content to an area of lower content •ΨT Can be measured in kPa, bars or atmospheres
  • 7. ΨT = Ψz+Ψs+Ψm+Ψp • Smaller negative number, the higher the water content in the soil pores • The larger the negative number, the lower content will be in the soil pores Water will moves from area of higher to lower concentration Water moves from area of a lower negative value forΨT towards to an area of a higher negative value forΨT. Or
  • 8. Soil water potential log scale • The sandy soil holding considerably less water than the loam soil which holding correspondingly less amount of total water than the clay (fine textured soil) • In all case as soil water potential increases or becomes a larger negative number the soil will end up losing water regardless of whether it’s medium or fine texture soil • The best thing to use this graphic to illustrate is what type of soil moisture is remaining at a given soil water potential value.
  • 9. At -0.1 bars/atmospheres to -1 bars/atmospheres: • The sandy soil is not holding very much water • The loam soil is holding an intermediate amount • The finer clay texture soil is holding the most water at that soil water potential • -1 bars/atmospheres is not a large negative number, it is high value and has a lot of usable water in the soil profile
  • 10. At -10 bars/atmospheres • The water deficiency point for plants is usually around -10 bars/atmospheres and it is vary from one plant to another • It is fairly representative value that most of crop plants at least can take water out of soil profile without significantly damaging their yield potential • Everything’s going to this value it’s a combination of gravitational, matric and osmotic potentials
  • 11. Conclusion of suitable value of soil water potential for plants • For all the practical purposes, the level of water being held between -0.1 to -10 bars/atmospheres is really usable water that being stored in the soil plant. • At -10 to -100 bars/atmospheres is very little water left in the soil profile. Rarely plants are able survive and utilize water from soils down to this level.